Thin-film photovoltaic (PV) cells are usually characterized by a light-absorbing semiconductor such as amorphous silicon, copper indium gallium diselenide (CIGS), or cadmium telluride. This distinguishes them from the traditional crystalline silicon-based PV cells. Thin-film refers to the thickness of the semiconductor which is typically about 2 microns for the thin-film cells as opposed to 30-50 microns for crystalline silicon (c-silicon) cells. Another difference between thin-film and c-silicon PV cells is the temperature limitations involved. Thin-film cells must be processed at less than 200° C. as the semiconductor and/or the substrate used in thin-film cannot withstand high temperatures. The traditional c-silicon PV cells may be processed at temperatures up to 800° C. Thus, the use of a polymer thick film (PTF) conductive composition as the front-side (sun side) electrode grid/bus bar is required. PTF compositions themselves are only stable up to approximately 200° C. Additionally, PTF compositions usually do not lend themselves to soldering as this is done at temperatures of 200 to 260° C. Further, most, if not all current PTF electrode compositions do not wet well with solder and do not possess good adhesion to the solar cell after soldering.
It is therefore a primary objective of this invention to produce a solderable PTF conductive composition which adheres to the underlying substrate with reasonable adhesion even after soldering.